The present invention relates broadly to a centrifuge control apparatus, and in particular to a variable ratio dual speed control apparatus for large centrifuge units.
In the prior art, large centrifuge units required main drive control systems which utilizes a 3 phase, 120 pole synchronous motor for fine speed control of the centrifuge. The induction motor rotor which was switched as a function of RPM used resistors to limit current and maintain a high torque during acceleration and braking of the 650,000 slug-ft.sup.2 inertia load. Because of the wide RPM range requirements of the centrifuge, 5 RPM to 116 RPM, a variable frequency, variable voltage power supply was required to excite the synchronous motor stator. A solid state power supply, using silicon controlled rectifiers (SCR) to synthesize, a 3 phase stepped wave form, provided the drive for the synchronous motor armature.
The operation of this prior art system consisted of selecting the desired RPM (G level) on the frequency synthesizer and manually accelerating the main arm with 550 volts until the synchrascope indicated synchronous speed was reached. The 550 volts to the main arm induction motor was removed and the variable voltage alternator, controlled by the sustainer pot, was used to maintain in near synchronous speed while power was applied to the synchronous motor stator and field. After the synchronous motor was properly excited, the manual phase shifter potentiometer and the sustainer potentiometer was adjusted for proper load division between the two drive motors, at the same time reducing the main arm velocity and position error shown on the synchroscope. Once the velocity and position error were within allowable limits, .+-.0.0017 radian, position error and .+-.0.0008 radian/sec, velocity error) the synchronous motor was placed in the "synch mode" by replacing the 720 cycle/rev with the precision frequency. The motor was now free to operate at the load angle required to maintain synchronization, damping being provided through the accelerometer and lag network to stabilize the loop. This prior art system, however, has disadvantages, a few of which are as follows:
1. The power output of the variable frequency power amplifier was limited to 20 KW (rated at 220 KW) due to design problems in the commutator section of the amplifier.
2. The variable frequency power amplifier was very unreliable due to noise sources being coupled into the SCR gates which caused frequent current overloads due to incorrect voltage sequences being generated by the amplifier.
3. The system could not be used for speed control below 15 RPM because of excessive synchronizing time.
4. Above 15 RPM, the system is cumbersome to operate and slow to synchronize with times of 4 to 10 minutes.